# Concurrency demos

### From HaskellWiki

A large range of small demonstration programs for using concurrent and

MVars

forkIO

## A simple example of parallelism in Haskell

This little piece of code computes an approximation of Riemann's zeta function, balancing the work to be done between N threads.

import Control.Concurrent import Control.Concurrent.MVar import Control.Monad import Data.Complex import System.Environment zetaRange :: (Floating (Complex a), RealFloat a, Integral b) => Complex a -> (b, b) -> Complex a zetaRange s (x,y) = sum [ (fromIntegral n :+ 0) ** (-s) | n <- [x..y] ] cut :: (Integral a) => (a, a) -> a -> [(a, a)] cut (x,y) n = (x, x + mine - 1) : cut' (x + mine) size (y - mine) where (size, modulo) = y `divMod` n mine = size + modulo cut' _ _ 0 = [] cut' x' size' n' = (x', x' + size' - 1) : cut' (x' + size') size' (n' - size') getParams :: IO (Int, Int, Complex Double) getParams = do argv <- getArgs case argv of (t:n:s:[]) -> return (read t, read n, read s) _ -> error "usage: zeta <nthreads> <boundary> <s>" main :: IO () main = do (t, n, s) <- getParams childs <- zipWithM thread (repeat s) (cut (1, n) t) results <- mapM takeMVar childs print (sum results) where thread s range = do putStrLn ("Starting thread for range " ++ show range) mvar <- newEmptyMVar forkIO (putMVar mvar (zetaRange s range)) return mvar